This is a mutlicenter pilot trial testing the safety tolerabiltiy and efficacy of cidofovir for PML in the setting of HIV infection. Diagnosing must be firmly established by accepted testing of PCR and clinical presentation or biopsy. Treatment will include maximal antiretroviral therapy with the addition of cidofovir in the doses used in AIDS patients to treat CMV retinitis. Efficacy will be judged by safety, tolerability and measures of PML (quan JC PCP on GSE, brain MR lesion change over 24wks.